For precision machining of parts in electronics industry, and the like, a laser machining technology has been gradually developed to be an ultra-precise, ultra fast, and large area machining technology. In particular, for machining of parts in a micro electronics industry field including a semiconductor, a display, a solar cell, a next generation high value added/high functional PCB, a next generation packaging industry, and the like, ultra-precise machining is essential.
For micro-sized ultra-precise machining, high-performance laser specifications are required. For micro machining, a laser of an ultraviolet region is used or femtosecond and picosecond pulse laser beams having a very short pulse width are used. At the same time, a high-quality laser, in which a spatial distribution of a laser beam is a single mode, is required. Further, for implementing the high speed and the large area, a high-repetition-rate, high-output pulse laser is required.
An example of a method for operating a laser in a pulse may include a Q switching method and a mode locking method. In case of laser diode, a method for directly modulating an applied current so as to be operated in a pulse is used.
A pulse having a pulse width from several nanoseconds to several microseconds may be generated by using the Q switching method and a pulse having a pulse width from several femtoseconds to several hundreds of picoseconds region may be generated by using the mode locking method. The laser diode may be operated to enable a continuous wave in a pulse having a pulse width of several hundreds of picoseconds depending on the current modulation. A high-output pulse laser system having a high-quality laser beam uses a master oscillator power amplifier (MOPA) type which is configured to include a low-output, high-quality pulse laser resonator and an amplifier amplifying an output of the resonator to a high output.
In this case, the amplifier is configured of a single stage or a multi-stage depending on a magnitude of a final output. Recently, a low-output laser diode (LD) is used as the pulse generator and a structure in which an output of the laser diode (LD) is amplified by a multi-stage optical fiber amplifier or a laser crystal amplifier so as to be amplified to a high output is used. Generally, the high-output, high-quality laser is operated in a near infrared (NIR) region by using a laser gain medium containing rare earth ions (representatively, Nd, Yb, Er, Tm ions, and the like). The high-output, high-quality pulse laser in a visible or ultraviolet (UV) region has mainly used a method for wavelength-converting a near infrared laser output into a nonlinear photonic crystal so as to be output.
FIG. 1 is a diagram illustrating a burst mode operating method of laser according to the related art.
Referring to FIG. 1, modulation of output light which is not wavelength-converted by a nonlinear wavelength conversion and optical separator 4 via a multi-amplifier including a plurality of amplifiers 3-1, . . . , 3-N from a laser generator 2 operated in a wavelength conversion laser using a nonlinear photonic crystal, that is, a pulse may be operated in various types of burst modes by controlling an intensity of an output pulse using an optical modulator 6 before a final output terminal or a nonlinear wavelength converter.
However, the method needs to use the high-output optical modulator to control a light intensity of the final output terminal. The high-output optical modulator may be expensive and difficult to perform a high speed operation.
To solve the above disadvantages, a method of simply modulating a low-output input pulse as illustrated in FIG. 2 is disclosed. Referring to FIG. 2, a method for simply on-off modulating an optical pulse of a laser diode or a pulse resonator used as a pulse generator 12 by operating a modulator 14 so as to modulate a pulse of a visible and ultraviolet wavelength which is final output light in a burst mode may be attempted.
However, as illustrated in FIG. 2, when a pulse is not input to a multi-amplifier including a plurality of amplifiers 13-1, . . . , 13-N from the pulse generator 12 for a predetermined time, energy is continuously accumulated in an amplifier medium. Therefore, when a subsequent pulse is input, a laser pulse is very strongly amplified, and thus the amplifier medium is damaged due to the amplified pulses P1 and P2. The pulse passing through the nonlinear wavelength converter is also output to the pulses P1 and P2 in the amplified state.
FIG. 3 is a graph illustrating a relationship between the energy accumulated in the amplifier medium and a saturation time when excitation energy is continuously applied from the amplifier.
Referring to FIG. 3, the energy accumulated for a predetermined time, that is, till the saturation time is continuously increases. The saturation time is changed depending on the amplifier medium, but in case of rare earth ions, generally ranges from several microseconds to several milliseconds.
FIG. 4 is a graph illustrating characteristics of the energy accumulated in the amplifier medium and an input optical pulse and an output optical pulse, when the input optical pulse is periodically input, amplified, and output, with continuously applying the excitation energy from the amplifier.
Referring to FIG. 4, when an input pulse of the amplifier and an output pulse of the amplifier are uniformly input, a distribution graph of energy to time also forms a uniform profile.
Hereinafter, a relationship between the input pulse and the output pulse of the amplifier output from the pulse generator will be described in more detail.
Referring to FIG. 5, in a pulse laser system having a MOPA structure, the low-output pulse generator has a repetition rate of a predetermined region and periodically generates the optical pulse. Generally, the excitation energy is continuously applied to the amplifier in a high-repetition-rate laser system uniformly.
Recently, a high-output laser diode having a continuous wave is generally used. The temporally uniformly applied energy is accumulated in the amplifier medium over time.
Most of the accumulated energy is consumed to amplify the input optical pulse. After the optical pulse amplification, the energy is again accumulated till the subsequent optical pulse is input and when the subsequent optical pulse is input, the energy is consumed to amplify the optical pulse again.
When the optical pulse is periodically input and the input optical pulse is not input for a predetermined time (t1 and t2 of FIG. 5), the energy accumulated in the amplifier medium is very large (E1 and E2 of FIG. 5) and when the optical pulse is input to the amplifier in this state, the optical pulse is very largely amplified (P1 and P2 of FIG. 5). The very largely amplified optical pulse leads to instability of an output of a laser system and causes damage to the amplifier medium and the optical system.
Therefore, simply on-off modulating and amplifying the optical pulse of the pulse generator leads to the instability of the output of the laser system and causes damage to the amplifier medium and the optical system after the amplifier terminal.